A research team led by Rice University physicist Frank Geurts has successfully measured the temperature of quark-gluon plasma (QGP) at various stages of its evolution, providing critical insights into a state of matter believed to have existed just microseconds after the big bang, a scientific theory describing the origin and evolution of the universe. The findings were published in Nature Communications Oct. 14.
The study addresses the long-standing challenge of measuring the temperature of matter under extreme conditions where direct access is impossible. By using thermal electron-positron pairs emitted during ultrarelativistic heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York, the researchers have decoded the thermal profile of QGP.
Temperature measurements existed previously but have been plagued by several complications such as whether they were of the QGP phase or biased by a Doppler-like effect from the large velocity fields pushing such effective temperatures.
“Our measurements unlock QGP’s thermal fingerprint,” said Geurts, a professor of physics and astronomy and co-spokesperson of the RHIC STAR collaboration. “Tracking dilepton emissions has allowed us to determine how hot the plasma was and when it started to cool, providing a direct view of conditions just microseconds after the universe’s inception.”
New thermal window into nuclear matter
The properties of QGP, a deconfined state of quarks and gluons, depend heavily on its temperature. Prior methods lacked the resolution or penetrating power needed to measure QGP’s inner thermal conditions without being affected by the evolution of this rapidly expanding system. With temperatures expected to exceed trillions of Kelvins, scientists needed an unobtrusive thermometer to capture real-time values.
“Thermal lepton pairs, or electron-positron emissions produced throughout the QGP’s lifetime, emerged as ideal candidates,” Geurts said. “Unlike quarks, which can interact with the plasma, these leptons pass through it largely unscathed, carrying undistorted information about their environment.”
However, detecting these rare pairs amid a sea of particle debris required unprecedented sensitivity and data fidelity, Geurts said.
Experimental breakthrough
The researchers employed a refined detection apparatus at the RHIC, calibrating their systems to isolate low-momentum lepton pairs.
They tested the hypothesis that the energy distribution of these pairs would provide a direct measure of QGP temperature. This technique, referred to as a penetrating thermometer in theoretical discussions, integrates emission data over the plasma’s lifetime, producing an average temperature profile.
The research team achieved a precise measurement despite the technological limitations of statistical data and difficulties in isolating background processes that could mimic thermal signals.
Key findings, implications
The study revealed two distinct average temperatures depending on the mass range of the dielectron pairs: a lower temperature of approximately 2.01 trillion Kelvin in the low-mass region, predicted by theoretical models and consistent with freeze-out temperatures from hadronic probes, and a significantly higher temperature of about 3.25 trillion Kelvin in the higher pair mass region.
This difference indicates that thermal radiation from the low-mass range, which creates these dielectrons, is predominantly emitted later near the phase transition. In contrast, those from the higher mass range originate from the earlier, hotter stage of the QGP’s evolution.
“This work reports average QGP temperatures at two distinct stages of evolution and multiple baryonic chemical potentials, marking a significant advance in mapping the QGP’s thermodynamic properties,” Geurts said.
By precisely measuring the temperature of the QGP at different points in its evolution, scientists gain crucial experimental data needed to complete the “QCD phase diagram,” which is essential for mapping out how fundamental matter behaves under immense heat and density, akin to conditions that existed moments after the big bang and are present in cosmic phenomena like neutron stars.
“Armed with this thermal map, researchers can now refine their understanding of QGP lifetimes and its transport properties, thus improving our understanding of the early universe,” Geurts said. “This advancement signifies more than a measurement; it heralds a new era in exploring matter’s most extreme frontier.”
Co-authors of this study include former Rice postdoctoral associate Zaochen Ye, now at South China Normal University; Rice alumnus Yiding Han, now at Baylor College of Medicine; and current Rice graduate student Chenliang Jin. Geurts’ U.S. Department of Energy Office of Science Award supported the study.